Stainless steel materials having a metal microstructure composed of a double phase, that is, a ferrite phase and a martensite phase and therefore having improved workability have conventionally been put on the market as high-strength stainless steel materials. Such stainless steel materials are produced by heat treatment for obtaining a microstructure having a double phase of ferrite and martensite. Since these stainless steel materials have two phases, they have both high mechanical strength derived from the hard martensite phase and good workability derived from the soft ferrite phase. It is therefore possible to produce a stainless steel material having, to some extent, excellent workability by the formation of a double-phase metal microstructure, but impossible to improve the workability further without limitation. It is therefore difficult to use a conventional stainless steel as a material of products required to have higher workability.
With a view to providing a high-strength stainless steel sheet excellent in ductility and well balanced between strength and ductility and therefore suited for applications to be subjected to bending work, Patent Document 1 discloses a production process of a high-strength stainless steel sheet comprising successively carrying out a step of heating a stainless steel sheet at a temperature in a two-phase region and then cooling it at a cooling rate of 5° C./s or greater, a step of cold rolling the resulting sheet at a predetermined rolling reduction, and a step of heat treating the sheet at a predetermined temperature.
With a view to improving workability, particularly, bending workability, Patent Document 2 proposes a method of moderately decarburizing the surface portion of a stainless steel sheet having a two-phase microstructure. This method can realize high bending workability because a soft ferrite phase can be formed much in the surface portion of a stainless steel sheet and ductility on the surface portion can be improved.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-323960    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-234290